Rotel RA-920AX

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Rotel RA-920AX High DC offset

Hey guys, a few weeks back I scored a whole Rotel component setup for free. The amp is a Rotel RA-920AX. When I first got it, one of the inputs didn't work properly, and the amp sounded rubbish, so I did some investigating.
I found that one of the fuses just after the secondaries on the transformer was blown, and blew straight away when replaced. I traced the fault to a faulty bridge rectifier, which I replaced and the fuse no longer blew. The amp sounds better but still sounds a bit shoddy, so I checked the DC offset, and it's right up around 88mV on both channels!?!

There is no DC offset adjustment on this amp, only bias.

What's likely to be the cause of this high offset? I realised that I'd set the bias current way too high a few days ago as I had the multimeter set to the wrong scale
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Could I have ruined the transistors? The amp works fine, but honestly sounds crap.

So far it owes me nothing, and is a good learning project as I'm a complete noob to amplifiers
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. It's in excellent physical condition so I wouldn't mind spending a few dollars fixing it up.
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what might cause some offset is leacky electrolytics ( small leakadge will or can do that ) but it can be also cause by a number of other reasons

it could be wise to tatake samples of electrolytics to measure and if you find any probs start replacing them all ..

on the other hand if amplifier has been expeosed to humid or thermal stress there is also a chance that cheap carbon resistors have drifetd from the original value ....

Generally in an old machine there is a procedure and a number of things to test and measure to bring back to life and real performance

Given the oportunity you may also upgrade some parts inside to gain better sound and stability ....

here is a tutorial about amplifiers http://www.diyaudio.com/forums/solid-state/136261-vintage-amplifier-repair-upgrade-manual.html

then again agood start will be a schematic that from a first look seems to be hard to find

regards sakis
 
The amp works fine, but honestly sounds crap.

So far it owes me nothing, and is a good learning project as I'm a complete noob to amplifiers
smile.gif
. It's in excellent physical condition so I wouldn't mind spending a few dollars fixing it up.

If you've got patience and a few hours, you could really make it into something a bit special by fixing the grounding. But this is not a job for the faint hearted - if you'd like I can give you some pointers where the grounding is messed up. Rewiring the grounds is not expensive, just fiddly and time-consuming, but in my own experience where I've done this the sonic benefits are worth the time and effort.
 
If you've got patience and a few hours, you could really make it into something a bit special by fixing the grounding. But this is not a job for the faint hearted - if you'd like I can give you some pointers where the grounding is messed up. Rewiring the grounds is not expensive, just fiddly and time-consuming, but in my own experience where I've done this the sonic benefits are worth the time and effort.


I'd be very interested in doing this if you have the time to tell me! :)

I need a new project so this would be a good one :)
 
I have the same amp (also given it along with a RDC-975 which I have already fixed the draw on) and the same issues with the DC. So far I have replaced 8 of the caps and noticed a little change but for me the mV are still around 95 & 70. I will be replace the remainder soon when they arrive along with the differential pairs. Hopefully all goes well, but I would also be very interested in fixing the grounding.

interested to see how well you get on tooch, this amp seams like it is worth the effort. I have had a listen to mine and was very pleased and that was before I stared repair
 
I'd be very interested in doing this if you have the time to tell me! :)

OK then that's a deal:D Your side of the bargain is to report back on how it changes the sound!

I need a new project so this would be a good one :)

I'll need a little time to print out the layout and schematic from the kindly provided service manual, annotate them and scan them back in. Or perhaps I could find a way to do it electronically. Stay tuned!
 
I have the same amp (also given it along with a RDC-975 which I have already fixed the draw on) and the same issues with the DC. So far I have replaced 8 of the caps and noticed a little change but for me the mV are still around 95 & 70. I will be replace the remainder soon when they arrive along with the differential pairs. Hopefully all goes well, but I would also be very interested in fixing the grounding.

interested to see how well you get on tooch, this amp seams like it is worth the effort. I have had a listen to mine and was very pleased and that was before I stared repair

I'm going to price up all of the caps today and also do a quick check of the resistors, to see if I should replace all of them at the same time.

I think it's worth the effort too!

Looks identical to the circuit of my RA-930AX. I'd be wondering why the bridge rectifier fried.

It might be worth checking the regulators for the opamps, and the opamps themselves - it's possible that the faulty bridge rec has caused some damage.

I desoldered the 776's and 896's and gave them a quick check with the diode check on my DMM, all appeared ok. Is there a more conclusive test I can do on them?

OK then that's a deal:D Your side of the bargain is to report back on how it changes the sound!


I'll need a little time to print out the layout and schematic from the kindly provided service manual, annotate them and scan them back in. Or perhaps I could find a way to do it electronically. Stay tuned!

Excellent, looking forward to it :D

I'll try and get the amp sorted to a 'baseline' level first, so I can see how it's meant to sound stock. Then I'll do the mods and report back!
 
Looks identical to the circuit of my RA-930AX. I'd be wondering why the bridge rectifier fried.

It might be worth checking the regulators for the opamps, and the opamps themselves - it's possible that the faulty bridge rec has caused some damage.

rotel has a history of blowing rectifiers on many models but most of times you just replace them and go
 
well its rather obvious Jaycee ... rotel is a decent constructor ...machines they've made of that series and other similar play way above average or lets say from the same range of other or similar products

they do have proper trafos and they do have enough capacitance for their circuits ,often also one trafo but two power supplies

while they have bearly enough ...or almost enough rectifiers and no soft start circuit ...so that will complete the picture for you ..... one power off and one power on that could be also be a loose plug/socket or a mains instand power loss and there it goes ....

i had replaced many in the past ... the point is that in other manufacturer either the trafo is too small to provide enough to blow the rectifier or caps to be charged are also not a heavy load to the rectifier since these are small in capacitance anyway ...

kind regards sakis
 
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adjusting ???means exactly what ??? these are probably fixed resistors what to adjust???

you probably mena to replace with something else that will give other type of ofset ...or replace them with a trimmer ..that could be a more realistic option

best thing will be to first verify all voltage arround there to se what is going on .... there might be a chance of something else also .... for example whta is your normal voltage down there ??? or down under ??? 220 or 240 ???

it could be that the amp is setup for 220 this lilte extra voltage might or could bring the amp outof balance ...so verify voltage before everything else ...also from the schematic to the testing points on the pcb ...

then verify close to ltp resistors for drifting .... if any suspect replace them all with metal film 1% ...that is also some type of upgrade


one other thing iwould like to do and since pcb and parts location helps is to join the ltp tranisotrs for thermal junction ...that also might solve the problem


regards sakis
 
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